1. Field of Invention
The field of the present invention relates in general to an adaptive pre-distortion systems and more particularly to a method and apparatus for adaptive pre-distortion in particular but not exclusively of X-DSL line drivers.
2. Description of the Related Art
North American Integrated Service Digital Network (ISDN) Standard, defined by the American National Standard Institute (ANSI), regulates the protocol of information transmissions over telephone lines. In particular, the ISDN standard regulates the rate at which information can be transmitted and in what format. ISDN allows full duplex digital transmission of two 64 kilo bit per second data channels. These data rates may easily be achieved over the trunk lines, which connect the telephone companies' central offices. The problem lies in passing these signals across the subscriber line between the central office and the business or residential user. These lines were originally constructed to handle voice traffic in the narrow band between 300 Hz to 3000 Hz at bandwidths equivalent to several kilo baud.
Digital Subscriber Lines (DSL) technology and improvements thereon including: G.Lite, ADSL, VDSL, HDSL all of which are broadly identified as X-DSL have been developed to increase the effective bandwidth of existing subscriber line connections, without requiring the installation of new fiber optic cable. An X-DSL modem operates at frequencies higher than the voice band frequencies, thus an X-DSL modem any operate simultaneously with a voice band modem or a telephone conversation. Currently there are over ten discrete X-DSL standards, including: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc.
One of the factors limiting the bandwidth or channel capacity of any of the above discussed X-DSL protocols is interchannel interference. Electronic amplifiers and line drivers employed in many communication systems inherently distort signals as they amplify them. Furthermore, to maximize power efficiency these line drivers are often operated near the saturation point where the input/output power characteristics become nonlinear. Amplitude modulation causes distortion to become dependent on the input signal with a result of the amplified output signal is no longer simply an amplified replica of the input signal. Unfortunately if linear modulation with a fluctuating envelope is used in conjunction with nonlinear amplification, spectral spreading into adjacent cans will occur thereby interfering with communications.
What is needed are approaches to reducing in band distortion and out-of band interference for X-DSL line drivers.